The Inertial Upper Stage or IUS is a two-stage solid-fueled booster rocket developed by NASA and the U.S. Air Force for the launching of large payloads from either a Titan III (later Titan IV) rocket or from the payload bay of the Space Shuttle.

Development of the IUS

During the development phase of the Space Shuttle (1969-1974), NASA, with reluctant support from the Air Force, wanted an upper stage that can be used on the Space Shuttle, but at the same time, can be switched over to the Titan III rocket (then the most powerful unmanned rocket in the U.S. fleet, since the Saturn INT-21, a derivative of the Saturn V rocket, was only used once for the launch of Skylab in 1973), in the case the Shuttle ran into lengthy delays either from development or testing. Although NASA wanted to adopt a version of the Centaur upper stage for its planetary missions, the Air Force wanted to use the "Transtage," a hypergolic upper stage used on most Titan III launches in which the Centaur was not needed, and unlike the Centaur, used the same fuel and oxidizer used on the Shuttle OMS and RCS systems.

The solid-fueled IUS was created as a compromise between the Transtage, which was not powerful enough for most NASA payloads, and the Centaur, which was not needed for all military and intelligence payloads. IUS was powerful enough to deliver two large DoD or NSA satellites into proper orbits over the former Soviet Union, or a single NASA payload (most notably the TDRS (Tracking and Data Relay Satellite) constellation into a geostationary orbit on either the Shuttle or the Titan III.

IUS's first launch was in 1982 on a Titan 34D rocket from the Cape Canaveral Air Force Station shortly before the STS-6 mission. Boeing was the primary contractor for the IUS Chemical Systems Division of United Technologies built the IUS solid rocket motors

IUS Function

The IUS is a two-stage rocket motor which operates in two phases. On Titan launches, the Titan III or IV booster would launch the payload into a low earth orbit, upon which the stage and its payload is separated. On Shuttle launches, the IUS and payload is raised to a 50° angle, upon which it is then released and then the Shuttle separates from the payload to a safe distance, much like that employed on the spin-stabilized commercial satellites and their attached Payload Assist Module (PAM) "kick motors." The IUS first stage then ignites and, on most missions, will place the payload in an egg-shaped "transfer orbit", after which the first stage and interstage units are jettisoned. The second stage then fires to circularlize the orbit, after which it too is jettisoned, upon which the satellite will then start its mission.

Missions That Used the IUS

As of 2007, the following missions have used the IUS rocket, most of them from the Space Shuttle, especially after the Shuttle version of the Centaur upper stage was banned due to an after effect of the Challenger Disaster in 1986.

#	Launch Date	Launch Vehicle	Payload	Notes
1	October 30, 1982	Titan 34D	DSCS-II F16 DSCS-III A1	Mission successful despite telemetry loss for most of the flight.
2	April 4, 1983	STS-6	TDRS A	Second stage tumbled due to a control system failure. Over the period of several weeks, ground controllers used excess fuel in the TDRS to move it into proper orbit.
3	January 24, 1985	STS-51-C	Magnum 1	Classified DoD payload.
4	October 3, 1985	STS-51-J	DSCS-III B4 DSCS-III B5	Classified DoD payload.
5	January 28, 1986	STS-51-L	TDRS B	Destroyed due to Challenger disaster.
6	September 29, 1988	STS-26	TDRS C
7	March 13, 1989	STS-29	TDRS D
8	May 4, 1989	STS-30	Magellan
9	June 14, 1989	Titan IVA	DSP 14
10	October 18, 1989	STS-34	Galileo
11	November 22, 1989	STS-33	Magnum 2	Classified DoD payload.
12	October 6, 1990	STS-41	Ulysses
13	November 13, 1990	Titan IVA	DSP 15
14	November 15, 1990	STS-38	Magnum 3 or an SDS-2 Exact payload unknown	Classified DoD payload.
15	August 2, 1991	STS-43	TDRS E
16	November 24, 1991	STS-44	DSP 16
17	January 13, 1993	STS-54	TDRS F
18	December 22, 1994	Titan IVA	DSP 17
19	July 13, 1995	STS-70	TDRS G
20	February 23, 1997	Titan IVB	DSP 18
21	April 9, 1999	Titan IVB	DSP 19	Spacecraft failed to separate from the second IUS stage.
22	July 23, 1999	STS-93	Chandra X-ray Observatory
23	May 8, 2000	Titan IVB	DSP 20
24	August 6, 2001	Titan IVB	DSP 21
25	February 14, 2004	Titan IVB	DSP 22

Current status

Currently, because of the use of the more efficient Centaur upper stage on the Atlas rockets, including the new Atlas V, the IUS has been in effect placed into "semi-retired" status, although it may be used in the future to augment the Delta IV rocket or even the planned Shuttle Derived Launch Vehicle (the Ares I and Ares V rockets). Although, this is highly unlikely as the final IUS launch used the last IUS vehicle and the Boeing IUS team has been disbanded, making the future production of IUS vehicles quite costly.

References

External links

• Evolution of the Inertial Upper Stage Crosslink Winter 2003 (published by The Aerospace Corporation)
• Inertial Upper Stage at Federation of American Scientists